The present invention relates to circuits and methods for controlling common mode voltage, and more particularly to a circuit and method for stabilizing a common mode voltage in a differential circuit which generates an output current which is to be converted to a voltage.
As is known, a differential circuit represents a signal by the difference v.sub.d between two voltages, v.sub.p and v.sub.n : EQU v.sub.d =v.sub.p -v.sub.n ( 1)
The voltage about which this signal varies is the common mode voltage v.sub.cm which may be found from: ##EQU1##
One way to bias the common mode voltage of a signal is by providing a known reference current I.sub.cm through a load resistors R.sub.L, such as in the circuit illustrated in FIG. 1 where I.sub.d is the current related to the difference V.sub.d. This method is usually adequate so long as load resistors R.sub.L can both set the load impedance and define the common mode voltage.
When the common mode voltage must be forced to a desired common mode reference voltage V.sub.cm a common mode feedback circuit can be used, such as illustrated in FIG. 2. In this example, MOSFETs M1 and M2 are variably resistant loads (i.e., current sinks for the reference current I.sub.cm) which operate in conjunction with an operational amplifier control loop to stabilize the common mode voltage of the signal at V.sub.cm. As is apparent, one of the problems with this approach is that the common mode voltage setting-loop is about as large as the circuitry in the signal path. The use of this much real estate is undesirable, and a simpler, more compact circuit which is easily adapted to a variety of uses is preferred.
Accordingly, it is an object of the present invention to provide a novel circuit and method of stabilizing a common mode voltage which is both simple and adaptable to a variety of uses thereby obviating the problems of the prior art.
It is another object of the present invention to provide a novel circuit and method of stabilizing a common mode voltage in which the desired common mode reference voltage is provided directly to the control terminals of two transistors which provide variable resistances for stabilizing the common mode voltage of the circuit's output.
It is yet another object of the present invention to provide a novel circuit and method of stabilizing a common mode voltage in a differential circuit in which a desired common mode reference voltage is sensed between two load resistors which connect input terminals of two transistors and in which the sensed voltage is provided to the control terminals of the two transistors to control operation thereof.
It is still another object of the present invention to provide a novel circuit and method of stabilizing a common mode voltage for a transconductance or transresistance amplifier input stage.
It is a further object of the present invention to provide a novel circuit and method of stabilizing a common mode voltage for an automatic gain control circuit.
It is yet a further object of the present invention to provide a novel circuit for providing a stable common mode voltage having two transistors for receiving input currents at first operating terminals, two series-connected load resistors connecting the first operating terminals of the two transistors, and a connection from between the two load resistors directly to control terminals of the two transistors, whereby the average of the output voltages from the circuit is the stable common mode voltage.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.